#
#  MovieHasher.rb
#  Undertext
#
#  Created by Johan Lundström on 2009-01-29.
#  Copyright (c) 2009 Johan Lundström.
#

# Specification and more info: http://trac.opensubtitles.org/projects/opensubtitles
class MovieHasher

  CHUNK_SIZE = 64 * 1024 # in bytes

  def self.compute_hash(filename)
    filesize = File.size(filename)
    hash = filesize

    # Read 64 kbytes, divide up into 64 bits and add each
    # to hash. Do for beginning and end of file.
    File.open(filename, 'rb') do |f|    
      # Q = unsigned long long = 64 bit
      f.read(CHUNK_SIZE).unpack("Q*").each do |n|
        hash = hash + n & 0xffffffffffffffff # to remain as 64 bit number
      end

      f.seek([0, filesize - CHUNK_SIZE].max, IO::SEEK_SET)

      # And again for the end of the file
      f.read(CHUNK_SIZE).unpack("Q*").each do |n|
        hash = hash + n & 0xffffffffffffffff
      end
    end

    sprintf("%016x", hash)
  end
end